1. Field of the Invention
The present invention relates to a multi-color ribbon cassette position adjusting apparatus which is used in a color printer for multi-color printing, and in which a multi-color ribbon cassette having a multi-color ribbon therein is shifted with respect to a print head.
2. Description of the Related Art
In general, a color printer is designed so that a multi-color ribbon cassette, having a multi-color ribbon therein, is shifted with respect to a print head to select ink colors on the ribbon for color printing. A multi-color ribbon cassette position adjusting apparatus is used for the selection of the ink colors on the multi-color ribbon. Conventionally, the position adjusting apparatus comprises a shifting mechanism for actually shifting the position of the ribbon cassette, with respect to the print head, and a fine adjustment mechanism for fine adjustment such that the ribbon is regularly positioned with respect to the print head.
Disclosed in Published Unexamined Japanese Patent Application No. 62-253480 is a multi-color ribbon cassette shifting mechanism, which is arranged in the manner shown in FIG. 1. The construction of this conventional shifting mechanism will now be described in brief.
A carrier 51, which holds a print head 50 thereon, is supported for movement along carrier shafts 56. A ribbon plate 53 for holding a multi-color ribbon cassette 52 is urged in a direction perpendicular to the moving direction of the carrier 51, and is shiftable in this direction. A cam shaft 57, which extends parallel to the carrier shafts 56, is provided with a cam 54 which moves together with the carrier 51. The ribbon plate 53 is fitted with a cam follower 55 which engages the cam 54. As the cam 54 is rotated by means of the shaft 57, the ribbon cassette 52, along with the plate 53, shifts its position in the direction of the arrow. The cassette 52 contains a multi-color ribbon coated with different ink colors which are divided in the direction of the arrow.
In this arrangement, a paper sheet traveling on a cylindrical platen 58, part of which is exposed from a table 60, is subjected to color printing by moving the carrier 51 along the carrier shafts 56 and shifting the multi-color ribbon cassette 52 for ink color selection in the direction of the arrow.
Constructed in this manner, however, the multi-color ribbon cassette shifting mechanism is subject to the following drawbacks.
The ink colors of the multi-color ribbon are alternatively selected by moving the multi-color ribbon cassette 52 in the direction of the arrow. In order to select required colors without fail, therefore, the cam shaft 57 for holding the cam 54 must be kept accurately parallel to the carrier shafts 56. In the shifting mechanism shown in FIG. 1, however, the shaft 57 receives the urging force of the ribbon plate 53 through the cam follower 55, so that it is liable to deflection.
If the parallelism of the shaft 57 to the shafts 56 is inaccurate due to this deflection, the position of the ribbon plate 53 relative to the carrier 51 varies, and the ink colors cannot be accurately shifted with respect to the print head 50, so that the paper sheet may possibly undergo color mixture printing. In order to solve this problem, it is necessary to adjust the parallelism of the cam shaft 57 to the carrier shafts 56, thus entailing higher cost.
In some cases, each color of the multi-color ribbon may not be able to be located in position with respect to the print head 50, due to machining or assembling errors of the plate 53, cam 54, etc. Accordingly, two color coat portions of the ribbon may simultaneously get hit by the print head 50, for example, possibly resulting in color mixture printing on the paper sheet.
Conventionally, therefore, the multi-color ribbon cassette position adjusting apparatus comprises a fine adjustment mechanism for finely adjusting the position of the multi-color ribbon cassette 52 with respect to the print head 50 in order to compensate the machining or assembling errors. However, no such adjustment mechanism is described in the aforementioned patent application.
FIG. 2 illustrates another conventional example of a multi-color ribbon cassette position adjusting apparatus which comprises a shifting mechanism for actually shifting the position of the multi-color ribbon cassette, with respect to the print head, and a multi-color ribbon cassette fine adjustment mechanism for fine adjustment such that the ribbon is regularly positioned with respect to the print head.
The shifting mechanism of this position adjusting apparatus is provided with a ribbon plate 53a for holding a multi-color ribbon cassette 52 which contains a multi-color ribbon 52a. A follower piece 75, which is formed at one end portion of the plate 53a, is in engagement with a helical guide portion 73 formed on a shift cam 71 which rotates around an axis 72. The other end portion of the ribbon plate 53a is pivotally supported on a carrier 51a so as to be rockable around pivots 62.
A drive motor 66, which is mounted on the carrier 51a, drives a train of gears 67 and 68 to rotate the shift cam 71. As the cam 71 rotates, the ribbon plate 53a, holding the multi-color ribbon cassette 52 thereon, is rocked around the pivots 62 through the medium of the helical guide portion 73 and the follower piece 75 in engagement with each other. By doing this, the position of the ribbon cassette 52 relative to the print head 50 is shifted.
Further, the shift cam 71 is provided with the multi-color ribbon fine adjustment mechanism 80, which serves to finely move the cam 71 in the direction of the axis 72. As the shift cam 71 finely moves in this manner, the ribbon plate 53a is slightly rocked around the pivots 62 through the medium of the helical guide portion 73 and the follower piece 75. By doing this, the multi-color ribbon 52a can be finely adjusted so that it is regularly positioned with respect to the print head 50.
FIG. 3 is a sectional view illustrating the arrangement of the multi-color ribbon fine adjustment mechanism in detail. This mechanism comprises a shift cam stud 81 for supporting the shift cam 71 for rotation and movement in the direction of the axis 72, and an adjusting nut 82 which threadedly engages a thread portion 81a of the stud 81 so that the position of the cam 71 with respect to the direction of the axis 72 is shiftable. The fine adjustment mechanism further comprises members (wave washer 84, shift cam washer 85, E-ring 86, and adjusting spring 87) for preventing vibration of the shift cam 71 in rotation, and a stabilizer nut 88 for preventing the nut 82 from being unexpectedly rotated by vibration or the like.
This multi-color ribbon cassette position adjusting apparatus, however, is subject to the following drawbacks. As shown in FIG. 3, the multi-color ribbon cassette fine adjustment mechanism has a complicated construction, including many components, so that it is difficult to reduce its size and cost.